Since every test pattern used in non-scan design circuit only needs one test cycle in testing, and non-scan design for testability provides an at-speed testing method, the method of non-scan design has gained attention from both academy and industry.
FIG. 1 shows the conventional test point structure. In particular, FIG. 1(a) shows an original circuit; FIG. 1(b) shows insertion of observation points; FIG. 1(c) shows insertion of 1-control point; and FIG. 1(d) shows the insertion of 0-control point.
The pin overhead in non-scan design has remained an unsolved problem for a long time and the previous methods cannot solve it successfully. Currently there are 6 methods to handle the pin overhead problem, but none of them can bring about a satisfactory result. These 6 methods are:
(1) doing nothing to the extra control signal, which leads to a strict restriction on the number of control points and cannot improve testability to a great extent;
(2) connecting the control points to an extra register, when loading the test pattern, the value to be assigned to control point is shifted into the register. However, the method needs multiple clock cycles to load the test pattern for control points.
(3) controlling all control points with one control signal. Almost all reset design methods have chosen this strategy, but testability improvement based on this technique is limited.
(4) setting the control signal of control points to non-controlling values in functional mode and to the controlling values in test mode. This method makes many detectable faults undetectable and hence can not improve the testability of circuit.
(5) using test multi-plexers to improve testability of circuit and all test multi-plexers are controlled by a single control signal, which is similar to method (3). Though it can improve the testability of some signal lines, many fault propagation paths are blocked, hence it cannot really improve testability of the whole circuit.
(6) connecting an observation point and a primary output (PO) to a multiplexer, which makes the fault that must be observed via this PO unobservable.
Therefore, the pin overhead in non-scan design is a very hard problem and needs to be solved as soon as possible.